Generally, satellite positioning systems (SPS), (e.g., the U.S. Global Positioning System (GPS) and the European Galileo System (currently under construction)), provide an invaluable service that has perhaps exceeded the imagination of the designers of the systems. For example, SPS systems are used in military, commercial, medical, scientific, and recreational applications.
When a SPS receiver is first turned on, it must be able to acquire and receive the signal transmitted from a certain number of SPS satellites before it can determine its position and the current time. For example, in the GPS system, the GPS receiver should be able to receive the transmitted signal from at least three or four GPS satellites. Each satellite in the SPS system transmits a unique signal that can be used by the SPS receiver (in conjunction with signals from other SPS satellites) to calculate the SPS receiver's position and the SPS time. One of the most vital pieces of information that is transmitted in the SPS signal is highly accurate timing information. The differences between the timing of the various SPS signals received by the SPS receiver and its own internal clock are then used to calculate the position of the SPS receiver and the current SPS time.
While SPS systems have become widely used today, there remains a major problem that hinders their use in certain situations. Since the satellites are in high-earth orbit and are typically powered by solar panels, the signals that are transmitted by the satellites are usually very weak by the time they reach the SPS receiver (signal strength being inversely proportional to distance). Additionally, SPS signals may be further attenuated by thick foliage, buildings, tunnels, etc. to a point where the SPS signals may fall below a minimum signal power threshold and become difficult or impossible for the SPS receiver to acquire and receive. Under such circumstances it may take several minutes or more to acquire the SPS signals, if acquisition is even possible.
It has been noted that if the SPS receiver can be provided with an accurate timing reference through a source other than the actual signals transmitted by the SPS satellites, the acquisition process can be accelerated or completed if it was previously unable to do so. Moreover, the more accurate the timing reference that is provided through an alternate source, the easier and faster the acquisition process becomes.